The Value of Bench-Scale Testing

The Value of Bench-Scale Testing


A Few Hundred Dollars Can Save Thousands of Dollars in the Field

Has this ever happened to you? Your budget and schedule is nearing the end, but you’re not close to the end. No one in our industry wants to be in this situation. At a recent meeting we were asked to conduct a third-party review of an on-going remediation program. Another contractor was hired to treat a petroleum-like compound in-situ. Contaminated groundwater was extracted, surfactants were added, and oxidant injections were completed. Yet the results were ambiguous – very little of the total contaminant mass had been removed.

This experience is the inspiration for this newsletter!

Nobody’s perfect and hindsight is 20/20, but we thought there were some basic questions that could have been asked early on in the remedial design that would have greatly increased the chances of success for the project. A better site conceptual model could have improved the remedial piping design and installation locations. A more in depth examination of the compounds of concern could have revealed that they were a novel mix of unusual compounds. With regards to remedial amendments, what was the influence of various surfactants and surfactant concentrations to contaminant mass removal, and what was the influence of various oxidants and oxidant concentrations on interactions with the aquifer materials and treatment of the compounds of concern? Prior to spending many hundreds of thousands of remediation dollars, had anybody conducted treatment testing in the lab?

In a perfect world it’s best to proceed with remediation in a systematic fashion: bench-scale, pilot-scale, then full-scale. At each stage the process is evaluated and optimized. If a test fails to produce the desired results in the lab, simply change the process at the bench-scale for only hundreds of dollars, and avoid improperly spending tens if not hundreds of thousands of dollars at full-scale.

It’s far less painful to make corrections at a small scale. And it’s easy to make changes at the bench- and pilot-scale levels.

Basic bench scale testing should be an easy sell to any client who is considering a full scale on-site remediation program. The costs are often small.

For example, we routinely test soils for a parameter known as natural oxidant demand (NOD). This is a test to assess how a particular soil responds to a particular oxidant. NOD is used to estimate how much oxidant may react with the site specific soil. It’s important as quite often the amount of oxidant mass lost during reaction with the soil is larger than that required to destroy the contaminant itself.
So what exactly is involved in a bench test?

Oxidant Injection for Consolidation Testing_Web

There are two basic types of bench-scale testing:
1.) Static batch reactors
2.) Flow-through column reactors

Static Batch Reactor tests typically take small samples of contaminated soil or groundwater (1 kg or 1 L each) from a site and expose them to various types and/or concentrations of remedial amendments in a sealed container. Treated sample results are compared to baseline or control sample results to assess treatment effectiveness. These tests are very quick and easy to perform and can provide valuable insight into reaction chemistry and dosing requirements.

Flow-Through Column Reactor tests are a bit more complicated and typically require a larger volume of site groundwater. Groundwater is passed through one or more columns containing various reactive media. Once again, treated sample results are compared to baseline or control sample results to assess treatment effectiveness. By their nature, these tests take a bit longer and cost a bit more to complete.

Either approach can be completed under aerobic or anaerobic conditions.

Recently we conducted static batch reactor testing on processes as varied as:

  • Contrasting Portland cement, lime, and phosphates to compare their ability to bind leachate hazardous lead into soils (the results were surprising!).
  • Dosing groundwater samples with organic substrates to encourage anaerobic bacteria to degrade PCE, TCE, DCE, and VC; this was tested with and without supplementary iron particles.
  • Precipitation of arsenic from groundwater under anaerobic and reducing conditions.
  • Assessing the effectiveness of the oxidant sodium persulphate with using different activation methods at treating pentachlorophenol (PCP) in soil.

Chemical Oxidation Test Samples_Web

As an example of a flow-through column test, we recently received a sample of wastewater from a testing facility.  The water was impacted with a wide range of organic and inorganic parameters, including:

  • Volatile organic compounds (VOCs);
  • Heavy metals;
  • Cyanide and formaldehyde;
  • Perfluorinated compounds (PFCs), including perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA); and
  • Semi-volatile organic compounds (SVOCs), including polycyclic aromatic hydrocarbons (PAHs).

One of the main contaminants was PFOS – an ingredient in fire-suppressing foams. PFOS is one of the so called “emerging contaminants”, known for their persistence and resistance to destruction by chemical, biological, or physical processes.

The standard practice of using vacuum trucks to haul this process water off site was proving very expensive. Theoretically a variety of traditional filtration media could effectively handle this cocktail of contaminants. But given the myriad of potential chemical and physical interactions, could we know the removal efficiencies?

To find out, we designed a series of flow-through columns containing a wide variety of filtration media. Each medium type was specifically selected for the removal of particular contaminant class and staged in a pre-determined order.

Sequential Flow Through Column Reactor Assembly_Web

The columns were packed, loaded with wastewater, and the residence time was monitored. The effluent of the sequential flow through-columns was then analyzed at various points through the system.

The final effluent results met the discharge criteria for all parameters, including PFOS and PFOA. The results of the bench-scale testing also led to a proactive decision to add a final activated carbon filter just prior to effluent discharge in the field for final polishing.

In addition, we have assessed varied and more complicated scenarios (site simulation testing), including:

  • Flushing biodegradable surfactants through pea gravel from an actual site to assess removal efficiencies of fuel contamination from a tank nest.
  • Assessing the removal efficiency of heavy metals from an acidic mine tailings plume under high groundwater flow velocity (short contact time) conditions using site groundwater and sulphate reducing bacteria.
  • Assessing the differential settlement potential of highly organic peat soils under simulated building loads when subjected to very high strength oxidant injections.

Anaerobic Batch Reactor Assembly_Web

Next time you encounter an unusual contaminant or site condition, consider whether it might be worthwhile completing some bench-scale testing first as a means towards a more efficient and cost-effective remediation program.